1. Field of the Invention
The present disclosure relates to an optical pickup and optical read/write apparatus that reads and writes data from/on an optical storage medium by irradiating its storage layer with light that has been emitted from a laser light source. More particularly, the present disclosure relates to an apparatus that verifies data being written on an optical storage medium by itself. Examples of optical storage media that can be used include an optical tape, an optical disc, and an optical card.
2. Description of the Related Art
Recently, the size of digital data that can be stored on a storage medium has been rising steeply year by year as the resolutions of video data and still picture data have been tremendously increased and as increasing numbers of paper media have been converted into electronic ones. Meanwhile, so-called “crowd computing” technologies that allow people to use various kinds of applications and services via servers and storage systems on some network have become more and more popular nowadays. According to such crowd computing technologies, as a huge number of users save various kinds of data on that storage system on the network, the amount of data accumulated there should keep on skyrocketing from now on.
In the meantime, as regulations have been established one after another with regard to the duty of preserving such a huge amount of data saved, it should also be increasingly important to devise a method for saving that enormous amount of data as securely and as reliably as possible.
An apparatus that writes data of such a huge size on an optical storage medium must perform the operation of seeing if data has been written just as intended on the optical storage medium in order to increase the reliability of writing. Such an operation will be referred to herein as a “verify operation”. In this description, an “optical storage medium” will refer to a medium on which a mark can be recorded optically when irradiated with a light beam. And the light beam is radiated from an “optical pickup” that includes a light source and an optical system that focuses the light beam emitted from the light source onto the medium. When the optical pickup irradiates an optical storage medium with a light beam, an irradiated portion of the optical storage medium comes to have a different optical property (such as a refractive index) from the other non-irradiated portion of the medium. Such an irradiated portion, of which the optical property has varied, will be referred to herein as a “recorded mark”.
In optical storage technologies, data can be read out from an optical storage medium by irradiating the storage medium with a relatively weak light beam with a constant intensity and detecting the light that has been modulated by, and reflected from, the optical storage medium. On a rewritable optical storage medium, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a base (which may be either a disc or a long film) on which grooves and lands are arranged. In writing data on rewritable optical storage medium, data is written there by irradiating the optical storage medium with a pulsed light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
In a recordable or rewritable optical storage medium, when data is going to be written on its recording material film, the recording material film is irradiated with such a light beam, of which the optical power has been modulated as described above, thereby recording an amorphous mark on a crystalline recording material film. Such an amorphous recorded mark is formed there by heating a portion of the recording material film that has been irradiated with a writing light beam to a temperature that is equal to or higher than its melting point and then rapidly cooling that portion. If the optical power of a light beam that irradiates the recorded mark is set to be relatively low, the temperature of the recorded mark being irradiated with the light beam does not exceed its melting point and the recorded mark will turn crystalline again after having been cooled rapidly (i.e., the recorded mark will be erased). In this manner, the recorded mark can be rewritten over and over again. However, if the power of the light beam for writing data had an inappropriate level, then the recorded mark would have a deformed shape and sometimes it could be difficult to read the data as intended.
To read or write data from/on an optical storage medium, the light beam always needs to maintain a predetermined converging state on a target track. For that purpose, a “focus control” and a “tracking control” need to be done. The “focus control” means controlling the position of an objective lens along a normal to the disc surface so that the focal point (or at least the converging point) of the light beam is always located on the target track. On the other hand, the “tracking control” means controlling the position of the objective lens perpendicularly to the track so that the light beam spot is always located right on the target track.
In order to perform such a focus control or a tracking control, the focus error or the tracking error needs to be detected based on the light that has been reflected from the optical storage medium and the position of the light beam spot needs to be adjusted so as to reduce the error as much as possible. The magnitudes of the focus error and the tracking error are respectively represented by a “focus error (FE) signal” and a “tracking error (TE) signal”, both of which are generated based on the light that has been reflected from the optical storage medium.
According to a known method, an optical pickup device for use to read and write data from/on a magneto-optical (MO) disc or an optical disc such as a Blu-ray disc (BD), a DVD or a CD can perform a verify operation while writing data using a zero-order light beam and ±first-order light beams that have been split by a diffraction grating. When such a method is adopted, the light beam that has been emitted from a laser light source is split through a diffraction grating and the storage layer of a given optical disc is irradiated with the zero-order and ±first-order light beams. Specifically, a write operation is performed using the zero-order light beam and a verify operation is performed using the ±first-order light beams. Such a technique is called “DRAW (direct read after write)”. With that technique adopted, data that has just been written is checked for any errors, thus resulting in higher processing rates and higher transfer rates. Such a technique is disclosed in Japanese Laid-Open Patent Publication No. 6-162532, for example.
The known method, however, takes into account neither how long it will take to complete the phase change right after a write operation has been done nor how much an aberration to be produced due to unexpected off-axis incidence of ±first-order diffracted light beams on an objective lens will affect. That is why the performance of the verify operation could be insufficient. For example, if a read signal for verification were generated while crystals right after a write operation has been done are changing their phase into an amorphous one, then the reflectance and the mark length would be slightly different from their expected values and the performance of the verify operation would decline. Likewise, if a read signal for verification were generated when there is significant influence of aberrations, the scanning performance would decline, too.
As for known magneto-optical discs, DVDs and CDs, an aberration produced due to such an unexpected off-axis incidence of light beams on an objective lens is tolerated to a certain degree. However, when a write operation needs to be performed on a BD that requires higher accuracy or on an optical tape with the optical disc technologies applied, the aberration produced will often be too significant to satisfy the required performance.
To overcome such a problem, the present disclosure provides an optical pickup device that will achieve stabilized verify performance with scanning linear velocities on an optical disc, an optical tape or any other optical storage medium taken into account.
Also, an embodiment of the present invention provides an optical pickup device that can minimize the generation of aberrations and will achieve stabilized verify performance with scanning linear velocities on an optical storage medium taken into account.